What are the implications of today’s attacks at Parliament for the terrorism threat level in Britain?

Right at this minute, the official threat level is already “severe”, meaning an attack is thought “highly likely” - something that has big implications for people living, working and visiting London. The only threat level above severe is “critical”, meaning a threat is thought to be imminent. For an attack to be thought imminent requires some specific information about a planned or expected attack.

The severe threat level was set in 2014 and is likely to remain in place for some years to come, certainly for as long as ISIS poses a viable threat to the West.

The severe level already provokes a very heavy response from Britain’s security services. Any recent visitor to London, especially around Whitehall, Parliament and tourist attractions, will have seen a huge police presence, with hundreds of officers bearing infantry-style assault rifles behaving as if they expect to find a terrorist around the next corner.

Such heavy security appears to have allowed the police to resolve today’s attack in very quick time.

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What level of response would we see if a terrorist attack was thought imminent and the threat level raised to critical?

The experience of the people of Brussels in November 2015 may give some indication. The city centre was effectively closed down for several days while police hunted for terror suspects believed to be on the run in the city. One security expert contacted by the ECHO said this afternoon it would be premature to say whether the security threat level will be raised.

More information will be needed before the Home Office and MI5’s Joint Terrorism Analysis Centre and the Security Service decide whether or not to change the threat level. In particular, police will want to know whether today’s attackers were acting in isolation or whether they are part of a wider network. If investigations suggest they are part of a wider network, then the terror threat could be raised.